Respiratory syncytial virus (RSV) is a leading cause of bronchiolitis and pneumonia among infants and children under 1 year of age (CDC National Center for Infectious Diseases (2004) Respiratory Syncytial Virus). RSV can also be a significant lower respiratory tract pathogen in immuno-compromised adults and the elderly. Individuals can be infected multiple times as natural infection with RSV does not induce protective immunity.
RSV is a negative-sense, single-stranded RNA virus that belongs to the genus Pneumovirus of the family Paramyxoviridae. The RSV genome is surrounded by a helical nucleocapsid and encodes at least ten proteins: three transmembrane structural proteins (F, G, and SH), two matrix proteins (M and M2), three nucleocapsid proteins (N, P, and L), and two nonstructural proteins (NS1 and NS2) (Collins et al (1996) Respiratory syncytial virus, pp. 1313-1351, In B. N. Fields (ed.), Fields virology. Raven Press, New York, N.Y.). Neutralizing antibodies appear to be elicited only by the F and G proteins. RSV is divided into subgroups A and B based on the G protein, whereas F is more closely related between the subgroups. Monoclonal antibodies against the F protein have been shown to have neutralizing effects in vitro and prophylactic effects in vivo (e.g. Anderson et al. 1988. J. Virol. 62:4232-4238; Anderson et al. 1986. J. Clin. Micro. 23:475-480; Beeler and Coelingh 1989. J. Virol. 63:2941-50; Garcia-Barreno et al. 1989. J. Virol. 63:925-32; Taylor et al. 1984. Immunology 52: 137-142; and U.S. Pat. No. 6,818,216).
No safe and effective vaccine exists for RSV despite several decades of research. A formalin-inactivated virus vaccine tested in infants and children did not protect against infection and was associated with increased risk of severe symptoms during subsequent infections by wild-type RSV virus (Kapikian et al., 1969, Am. J. Epidemiol. 89:405-21; Chin et al., 1969, Am. J. Epidemiol. 89:449-63). Later attempts focused on developing live-attenuated temperature sensitive mutants also failed due to the inability to identify virus candidates at an appropriate level of attenuation and to the genetic instability of some candidates (I lodes et al. (1974) Proc. Soc. Exp. Biol. Med., 145, 1158-1164; Kim et al. (1973) Pediatrics, 52, 56-63; Wright et al (1976) J. Pediatrics, 88, 931-936).
Virus-like particles (VLPs) offer several advantages over conventional vaccine technology. An important advantage of VLPs for vaccine development is that they mimic native viruses in terms of three-dimensional structure and the ability to induce neutralizing antibody responses to both primary and conformational epitopes and therefore should prove more immunogenic than other vaccine formulations. Unlike viral vectored approaches, VLPs exhibit no problem with pre-existing immunity, thus allowing for recurrent use. VLPs containing RSV antigens have been generated by co-expression of RSV F protein with RSV matrix (M) protein or with a heterologous M protein in insect cells (US 2008/0233150). However, US 2008/02331050 does not teach actual expression of the RSV-F protein with a lentivirus or alpha-retrovirus GAG protein. In addition, the applicants in U.S. Ser. No. 61/115,780 teach expression of the RSV-F protein in the absence of an enveloped virus core forming polypeptides. However, the applicants found that the murine leukaemia virus (MLV—a gamma-retrovirus) GAG protein does not effectively form VLPs when co-expressed with the RSV-F protein. Thus, there is a need for VLPs that express RSV F protein on a core formed by a structural polypeptide other than the native RSV M protein.